The nickel-cadmium battery has been generally used as a rechargeable secondary battery. Recently, with the prevalence of smaller electric devices including cellular phones, personal-handy phone systems (PHSs) and mobile PCs, lithium ion batteries which are lightweight, smaller and less environmentally damaging than the nickel-cadmium battery are being widely used. Meanwhile, for reduced CO2 emission, hybrid vehicles, which use a gasoline engine and electric motor(s) as power sources, have been on the market. Hybrid vehicles use a nickel-hydrogen battery. For reduced manufacturing costs, research is being made on the development of high-performance lithium ion batteries that can replace the expensive nickel-hydrogen batteries.
A separator in a lithium-ion battery allows an electrolyte or ions to pass through while separating the cathode and anode from each other so as to avoid a short circuit between the electrodes. To achieve this, the separator is required to have various properties in electrical, chemical, and mechanical aspects. For example, in order to manufacture lightweight and small batteries, the separator needs to have sufficient mechanical strength even when it is made thin.
Moreover, the separator needs to meet particularly strict battery safety requirements. For instance, when a high current flowed due to an external short circuit, the separator should swiftly break the battery circuit. Currently, for lithium-ion battery separators, polyethylene microporous sheets are in practical use. These sheets are manufactured either by stretching or phase separation. The polyethylene microporous sheet melts at relatively low temperatures of heat generated due to a short circuit, thereby closing micropores to break the battery circuit and avoiding possible temperature increases in the battery after the closure of the micropores.
However, it is important that the separator offer not only excellent “shutdown characteristics” that allows its micropores to close when exposed relatively low temperatures, but excellent “shape retention” that allows the separator to retain its shape when exposed to high temperatures. Failure to retain the separator's shape at high temperatures brings the battery to a dangerous state of direct contact between the cathode and anode. For their low melting points, conventional polyethylene microporous sheets have the disadvantage of insufficient shape retention. To overcome this problem, separators are proposed in which a polyethylene film and a polypropylene film whose melting point is higher than that of the polyethylene film are laminated. These separators, however, have met with only limited success, with somewhat improved shape retention.
As a candidate for films that would offer excellent shape retention at high temperatures, porous films made of high-melting point polyolefin such as poly(4-methyl-1-pentene) or polypropylene have been studied.
As a method of obtaining a high yield of 4-methyl-1-pentene copolymer, a method of preparing 4-methyl-1-pentene copolymer in two stages using different amounts of α-olefin is disclosed (see, e.g., Patent Literature 1). The literature states that the method can produce a high yield of 4-methyl-1-pentene copolymer having high transparency.
Resin compositions containing 4-methyl-1-pentene polymer and 4-methyl-1-pentene copolymer are disclosed (see, e.g., Patent Literature 2).
As resin compositions suitable for battery separators, there are also disclosed resin compositions containing polyolefin resin and a nucleating agent, wherein the polyolefin resin consists of crystalline polypropylene and propylene-α-olefin copolymer (see, e.g., Patent Literature 3).